In recent years, enhancement of metal material properties such as strength and formability has been remarkable. Following this, there has risen a necessity to measure such properties of metal materials and a variety of other properties, which have strong correlations with the aforementioned properties of the metal materials, in the manufacturing process and inspection process.
For example, there is a strong correlation between the strength and the grain size of such a metal material, and it is known that, if the grain size is reduced to an approximate range from several 10 μm to 1 μm, a high-performance metal material which not only has high strength but also excellent processability and recyclability, is obtained. It is frequent that quality control of the metal material, which is as described above, is performed by a destructive test. For example, a test piece is cut out from a product coil already subjected to a rolling process, and strength thereof is confirmed by a tensile test. However, in such a method, the lead time until a measurement value is obtained is as long as several hours to several days, and defective products cannot be prevented from occurring continuously during this time.
To solve this problem, property measurement of the metal material by a laser ultrasonic wave method is attempted (for example, refer to PTL 1). The laser ultrasonic wave method is a method for measuring the metal material properties such as the grain size in a non-contact manner by irradiating a pulse laser beam onto a surface of the metal material and analyzing the propagation behavior of a pulse-like ultrasonic wave (hereinafter, referred to as a “pulse ultrasonic wave”) in the metal material, the ultrasonic wave having a high frequency generated at the time when the pulse laser beam is irradiated. In accordance with this method, measurement results are obtained rapidly.
The properties of the metal material, which are to be detected by using the propagation behavior of the pulse ultrasonic wave, are various, for example; the grain size, the elasticity, the formability (r-value), the elastic wave velocity and the like. Moreover, it is known that there are strong correlation relationships between the grain size of the metal material and tensile strength, yield strength and yield elongation, and thus values for these properties can also be obtained by a similar measurement method to that used to measure the grain size of the metal material.